In recent years, stringent requirements concerning the efficiency of power supplies have brought attention to the use of switched mode power supplies (SMPS). However, the actual efficiency of a SMPS depends on the controller that determines the switching instants of the one or more switches included in the SMPS. One approach for controlling the switching of an SMPS is using pulse width modulation (PWM), which can be implemented quite easily. However, using PWM for controlling a SMPS to regulate its output voltage or output current does not necessarily guarantee a high efficiency over a wide range of output currents.
A PWM based controller unit operates at a fixed frequency (PWM frequency). The duty-cycle of a rectangular PWM signal is modulated in order to regulate the power-supply output voltage or current. In some cases, the efficiency of a power converter decreases at low output currents as driving losses remain constant. These driving losses are related to the power consumed by switching on and off the one or more semiconductor switches used in the output stage of the switching converter. In order to decrease the contribution of driving losses to the total amount of losses, pulse frequency modulation (PFM) may be used for controlling the one or more semiconductor switches included in the output stage of the switching converter.
A controller unit using PFM reduces the duty cycle by reducing the switching frequency while keeping a constant on-time, instead of reducing the on-time at constant frequency, as it is done with PWM control. The efficiency of PFM control, however, becomes increasingly as the switching frequency and output current increases, thereby causing a corresponding increase in switching losses and driving losses.